This article is about an astronomy term. For other meanings, see zenith (disambiguation).
The zenith is the point in the sky which appears directly above the observer. More precisely, it is the point on the sky with an altitude of +90 degrees, and it is the pole of the horizontal coordinate system. Geometrically, it is the point on the celestial sphere intersected by a line drawn from the center of the Earth through your location on the Earth's surface. The point opposite the zenith is the nadir.
The zenith is, by definition, a point along the local meridian.
References
This article originates from Jason Harris' Astroinfo which comes along with KStars, a Desktop Planetarium for Linux/KDE. See http://edu.kde.org/kstars/index.phtml